Gilligan has led the Conference's efforts against Senate Bill 10, which changes the state's definition of marriage from "between a man and a woman" to "between two persons." The Thomas More Society is representing several downstate county clerks who have intervened to defend current state law defining marriage in a case brought by the American Civil Liberties Union and Lambda Legal against the Cook County Clerk for refusing to issue a marriage license to same-sex couples. Both the Cook County state's attorney and the Illinois Attorney general have declined to defend current state law. CCI filed an amicus brief in the case.

Harris is the lead sponsor of SB 10 in the House, and Mell is a former state legislator who also sponsored SB 10. The legislation passed the Senate on Feb. 14, but was not called for a vote in the House by the legislative session's May 31 deadline because it lacked the needed 60 votes for passage. SB 10 could be called for a House vote during the fall legislative session, Oct. 22-24 and Nov. 5-7.